Does Mickey Mouse exist in the DuckTales universe? (or even the general Duckverse). I don't mean hidden Mickeys or cameos, I mean referenced in a way that acknowledges his existence.
Both television series and comics are canon.
Not officially, no. In fact, Disney has a strict policy of not including him in any of the Ducktales universe. The current Disney XD reboot wanted to cross that line but the producer, Frank Angones, was told no
Why can't you have Mickey in Duck Tales?
He’s way above my pay grade. I had one story that involved him and one that mentioned him, but both were vetoed.
Don Rosa always drew the Duckverse cartoons in a different universe than Mickey (as far as I know). The above comic is the one exception I'm aware of. Since it was never published in America, I'm not sure if its part of Duck cannon. If there are any other examples please add them. (Interestingly, this is the one Disney comic I've ever seen that acknowledges the existence of the Loony Toons universe - i.e, Daffy Duck.
Does Mickey Mouse exist in the DuckTales universe? (or even the general Duckverse).
It's a bit complicated, here. First problem, there are many different universes/continuties in Disney; second you want to know about the existance, not the appearance, which is a detail that the current accepted answer fails to aknowledge.
The easiest bit of knowledge is the fact that Mickey Mouse and Donald Duck, in the "main Disney continuity" (usually referred as Mouseverse) are close friends. Actually in the main continuity all of the Disney comic characters exist, and crossovers are not so infrequent.
Outside of the main continuity the pairing of at least Mickey and Donald is a very common event: they have shared comic adventures, been together in countless movies (which are often "canon" and referenced decades later), videogames, and so on. The same applies to a plethora of other characters, even if it's a little less common.
So, in general we can sum it up to: